1,823 research outputs found
Flux Noise in MgB2 Thin Films
We have performed flux noise and AC-susceptibility measurements on two 400 nm
thick MgB films. Both measurement techniques give information about the
vortex dynamics in the sample, and hence the superconducting transition, and
can be linked to each other through the fluctuation-dissipation-theorem. The
transition widths for the two films are 0.3 and 0.8 K, respectively, and the
transitions show a multi step-like behavior in the AC-susceptibility
measurements. The same phenomenon is observed in the flux noise measurements
through a change in the frequency dependence of the spectral density at each
step in the transition. The results are discussed and interpreted in terms of
vortices carrying an arbitrary fraction of a flux quantum as well as in terms
of different macroscopic regions in the films having slightly different
compositions, and hence, different critical temperatures.Comment: 8 pages, 4 figures, conference contribution to "Fluctuations and
Noise", Santa Fe, New mexico 1-4 june 200
Growth of superconducting MgB2 thin films via postannealing techniques
We report the effect of annealing on the superconductivity of MgB2 thin films
as functions of the postannealing temperature in the range from 700 C to 950 C
and of the postannealing time in the range from 30 min to 120 min. On annealing
at 900 C for 30 min, we obtained the best-quality MgB2 films with a transition
temperature of 39 K and a critical current density of ~ 10^7 A/cm^2. Using the
scanning electron microscopy, we also investigated the film growth mechanism.
The samples annealed at higher temperatures showed the larger grain sizes,
well-aligned crystal structures with preferential orientations along the
c-axis, and smooth surface morphologies. However, a longer annealing time
prevented the alignment of grains and reduced the superconductivity, indicating
a strong interfacial reaction between the substrate and the MgB2 film.Comment: 7 pages, 4 figures include
The Dendritic magnetic avalanches in carbon-free MgB thin films with and without a deposited Au layer
From the magneto optics images (MOI), the dendritic magnetic avalanche is
known to appear dominantly for thin films of the newly discovered MgB. To
clarify the origin of this phenomenon, we studied in detail the MOI of
carbon-free MgB thin films with and without a deposited gold layer. The MOI
indicated carbon contamination was not the main source of the avalanche. The
MOI clearly showed that the deposition of metallic gold deposition on top of a
MgB thin film improved its thermal stability and suppressed the sudden
appearance of the dendritic flux avalanche. This is consistent with the
previous observation of flux noise in the magnetization.Comment: 9 pages, 4 figeure
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